Differential pressure is commonly used for the measurement of pressure, flow and level in industrial processes by measuring a pressure drop near a discontinuity within the pipe. The discontinuity may be in the form of an orifice, nozzle, Venturi, pitot tube, vortex shedding bar, target, or even a simple bend in the pipe. Impulse lines are used to couple a differential pressure sensor to the process on either side of the discontinuity, and provide two pressures (a high pressure, HP, and a low pressure, LP) from different locations in the process, such as on either side of the discontinuity, to a differential pressure transmitter. The differential pressure transmitter measures the difference in pressure between the two impulse lines, and creates and transmits a signal representative of the pressure difference.
Blockage in the impulse lines may occur because dirt or other material in the process may settle in the lines. Typical types of blockages include, for example, solid depositions, wax depositions, hydrate formation, sand plugging, gelling, frozen process fluid plugs and air or foam pockets. Such blockages may lead to erroneous measurement and undesired control actions based on the erroneous measurements. The impact of the erroneous measurements and control actions may result in poor control of the process, and catastrophic results. This can lead to a large loss of revenue, loss of life, and damage to property.
Periodic disassembly and inspection of the impulse lines is one method used to detect and correct plugging of lines. Another known method for detecting plugging is to periodically add a check pulse to the measurement signals from a pressure transmitter. The check pulse causes a control system connected to the transmitter to disturb the flow. If the pressure transmitter fails to accurately sense the flow disturbance, a signal may be generated indicative of plugging.
One prior art system detects plugged impulse lines by subtracting a moving average from a signal and then calculating a variance of the signal. A plugged line is then identified based on a threshold. Such techniques may be undesirable under varying process conditions Other prior methods utilize an absolute pressure signal. Generally, in some other techniques digital output from a differential pressure transmitter is sampled at random intervals and reconstructed into a time varying output that represents the process signal to produce samples. Plugging is detected based on whether variance of such samples exceed a predetermined threshold.
However, all techniques in related art generally focus on plugged impulse line detection. Such a focus may deteriorate the quality of decisions generating false diagnostic information.
Current methods do not address automatic detection of plugged impulse lines under various flow conditions. Under different flow rates with different fluids, false alarms may result, as well as missing actual plugged conditions.